I heard a story about a professor studying crows (or possibly ravens).

For an experiment he put on a Halloween mask and started harassing the birds until they'd swarm him and chase him away. Then he'd leave and come back without the mask and the birds would be pretty chill. A few days later he'd bring our the mask and he'd get swarmed again. I think he got some volunteers to wear the mask too, and they had the same experience. The birds were trained to hate masks.

Anyway, one academic career later and he finds the mask in a dusty office something like forty years later. He decides to try it again, and the birds swarm him.

The mask hadn't seen the light of day for decades by this point. My understanding was that none of the birds from the original experiment would have still been alive.

The implication that the professor arrived at was that these birds had folklore which they've passed down through the generations. So they're teaching their kids to hold onto their grudges even longer